Rugging a horse, eliminates the ability of the skin to take up Vitamin D from natural sunlight.
Direct sun exposure is the best way to absorb vitamin D. Recent studies demonstrate that with the absence of Vitamin D, alopecia can develop. https://www.ncbi.nlm.nih.gov/pubmed/17223342
Modern practises of rugging horses sometimes continuously may predispose the horse to suboptimal intakes of vitamin D. Restricting turn out time for sun exposure will fail the body having time to convert vitamin D in the skin. https://ker.com/equinews/vitamin-d-equine-diets/
Karen Langston for the National Association of Nutrition Professions says “Vitamin D maintains blood calcium levels and it regulates calcium and phosphorus, which keeps bones and teeth hard. Vitamin D deficiency – The biggest concern is softening of the bones… weak muscles, bone pain and tenderness”.
At trial low serum levels of vitamin D appear to be associated with an increased risk for progression of osteoarthritis of the knee. http://annals.org/aim/article-abstract/709914/relation-dietary-intake-serum-levels-vitamin-d-progression-osteoarthritis-knee
“Vitamin D plays an essential role in maintaining a healthy mineralized skeleton for most land vertebrates. Vitamin D keeps the serum calcium and phosphorus concentrations within the normal range to maintain essential cellular functions and to promote mineralization of the skeleton.” https://academic.oup.com/jn/article/126/suppl_4/1159S/4724783
Dr Juliet M Getty explains: “Reduced appetite, slowed growth, physitis in growing horses, bone demineralization (leading to stress fractures and bone deformities), and poor muscle contraction, are deficiency outcomes. Horses do best when they receive at least 6.6 IU of vitamin D per kg of body weight. For an 1100 lb (500 kg) horse, this translates into 3300 IU/day. Sunlight exposure – 5 to 8 hours/day – under optimal conditions, will produce this amount of vitamin D. http://gettyequinenutrition.biz/Library/VitaminDThesunshinevitamin.htm
More from Dr Getty “For a horse, the hair coat alone creates such a significant barrier to absorption that it typically takes five to eight hours of exposure to ultraviolet light for horses to produce enough vitamin D to satisfy the daily requirement. Compound that with additional barriers like rugging, fly spray, coat conditioners or decreased oils from bathing, it become apparent a horse may not be getting enough vitamin D.”
Frequent bathing with soap inhibits the body’s ability to produce vitamin D simply because the precursor (7-dehydrocholesterol) is washed away.
Dr Claire Thunes PHD suggests “those with limited exposure to sunlight get fed levels of vitamin D that meet the current NRC guidelines. You can achieve this by selecting a fortified commercial feed or supplement that provides about the guaranteed levels of vitamin D, and then feed the correct amount.”